1. Field of the Invention
The present invention refers to a container capable of preventing the formation of electrostatic charge, intended for the storage and/or transportation of liquids or powders, in particular inflammables, and also, but not exclusively capable of being used in environments with a high risk of explosion.
In particular, the invention regards a container for the storage and/or transportation of liquids or powders, in particular inflammables, suitable for preventing the formation of electrostatic charge, comprising a tank externally metallized, supported by a pallet and housed in a metallic cage so as to be in contact with it, as well as the method for carrying out the outer metallization of the tank.
2. Description of the Related Art
As is known, the presence of an outer conductive surface layer allows the rapid draining, towards a means connected to ground, of the possible static electricity which accumulates on the outer surface of the tank, for example during the moving of the container, the filling/emptying of the tank or in other circumstances in which any sort of friction is produced on the surface.
Moreover, it is necessary to foresee the possibility of using the tank in areas where there is a risk of explosion, for example chemical firms or areas intended for varnishing, where substances which can detonate are used and manipulated, even if the material inside the tank is not per se explosive.
In industry, plastic tanks are now widely used for storing and transporting liquid, powdered, granular and volatile products.
With respect to metallic tanks, plastic tanks have countless advantages, such as resistance to corrosion, ability to recover its original shape if subjected to deformations and thermal insulation.
To transport liquids plastic tanks are commonly used, commonly referred to as IBCs (Intermediate Bulk Containers) with capacities of between 450 and 3000 liters.
Such tanks are housed inside metallic cages supported by pallets consisting of simple wood, of plastic or of metal.
Whereas the tank carries out the task of containing the liquid, the metallic cage guarantees the necessary structural resistance, preserving the integrity of the tank in case of stresses due to knocks, falling and vibrations of the container.
In this way the container satisfies safety requirements both during warehouse storage, and during movement and transportation.
The problem is more complex in the case in which such containers are intended to contain and transport materials, in particular materials which are flammable and/or which have a high risk of explosion, and are moved, filled or emptied in explosion risk areas classified under R044-001 in the CENELEC Report (February 1999) Comité Européen de Normalisation Electrotechnique, Brussels.
In particular, amongst the flammable substances which plastic IBCs can contain it is necessary to consider those with an average and high flammability point, for example 3.2 and 3.3 according to RID ADR IMO.
Indeed, it is known that plastic tanks, for example made of high density polyethylene (HDPE), are, like all electrically insulated bodies, subjected to the accumulation of surface electrical charge by triboelectric effect during their handling, the loading and unloading of the material from the tank or by simple exposure to relatively dry air flows.
The electrical charge, or static electricity thus accumulated in turn generates around the tank an electric field the intensity of which can reach values so high, even only locally, by effect of the geometric shape of the tank or of surrounding elements (which in turn can be already charged or charged up by electrical induction/polarization), as to exceed the insulating strength of the environment (air or supports) surrounding the containers.
This can determine the growing up of electrical arcs, with the consequent risk of ignition of the vapors given off by the tanks, of the substances contained in them or of the vapors previously present in the external environment for example in the case of explosion risk areas.
Against the aforementioned advantages the plastic material which the tank is made of, which determines its electrical insulation, is the main cause of the formation of electrostatic discharges or electric arcs.
It is therefore necessary to prevent the accumulation of electrical charge on the surface of containers and, more specifically, of tanks providing them with suitable provisions which allow its easy dispersion to ground.
In the case of entirely metallic containers or tanks this is obtained by simply providing suitable ground connections for them.
Nevertheless, nowadays, the most widely used containers are those which comprise a plastic tank not just because they are more cost-effective and handy but also for a better and wider-ranging compatibility with the substances which they have to contain.
For this kind of electrically insulated containers there is a strong need to prevent the formation of electrostatic charge, for example through coating of the outer surface of the tank with a conductive material, fully adherent or even just in contact, which can be connected to a ground.
The coating can be continuous or discontinuous, with more or less compact meshes, provided that they are such as to ensure a low surface resistivity.
Amongst the most recent tendencies of technology there is that of directly forming, on the outer surface of the containers, a highly conductive layer.
Various methods are known in the state of the art for forming conductive layers on the surface of containers intended for the storage and transportation of dangerous and flammable materials and for the handling of the containers themselves in high explosion risk areas.
As a replacement for the coating with conductive varnishes, which has the drawback of not ensuring a sufficiently low resistivity, and of being very degradable and flaky in time, it has been proposed in document EP 674,470 to form a conductive layer sintering metallic powders on the surface of a plastic tank.
By sintering, in brief we mean a process in which a metallic powder, specifically zinc and/or copper, is sprayed on the surface of the container and at the same time the surface is heated or treated by a flame, so that the surface melts and incorporates the metallic powder. Varnishing and galvanic plating methods of plastic materials also exist.
The aforementioned methods have the drawback of not providing a uniformly conductive surface, of requiring a substantial waste of electrical energy necessary for the operation of the heat generating devices, of producing a large amount of harmful waste which needs to be disposed of and hence structures and equipments suitable for such a purpose with a consequent increase in costs for the industry.
It must be highlighted that the thickness of the conductive layer obtained with known techniques is in the order of millimeters and allows only a slight electrical conductivity to be obtained.